Tensions continue to escalate in the aftermath of the alleged assassination of an Iranian nuclear scientist. Iran reportedly sent a threatening letter blaming the United States and other Western nations, and American government officials are urging Israel to hold off on a potentially disastrous airstrike.

A car bomb killed 32-year-old Mostafa Ahmadi-Roshan and his driver Wednesday morning. Both Secretary of State Hillary Clinton and Israeli President Shimon Peres have denied responsibility and condemned the attack on behalf of their countries, but according to a Reuters report, Iran isn't convinced.

From Reuters:

"We have reliable documents and evidence that this terrorist act was planned, guided and supported by the CIA," the Iranian foreign ministry said in a letter handed to the Swiss ambassador in Tehran, state TV reported. The Swiss embassy represents U.S. interests in a country where Washington has no diplomatic ties.The spokesman for Iran's Joint Armed Forces Staff, Massoud Jazayeri, said: "Our enemies, especially America, Britain and the Zionist regime (Israel), have to be held responsible for their actions."Iran in the past has accused Israel of causing a series of spectacular and sometimes bloody mishaps to its nuclear programme. Israeli officials do not comment on any involvement in those events, although some have publicly expressed satisfaction at the setbacks.Feeling the heat from unprecedented new sanctions, Iran's clerical establishment has brandished its sword by threatening to block the main Mid-East oil shipping route, starting to enrich uranium at an underground bunker and sentencing an Iranian-American citizen to death on spying charges.State TV said a "letter of condemnation" had also been sent to Britain, saying the killing of Iranian nuclear scientists began after the head of Britain's MI6 spy service announced intelligence operations against states seeking nuclear weapons.

Swallowing its pride over the accusations, the U.S. is focusing on discouraging Israel from taking retaliatory action against Iran.

President Barack Obama, Defense Secretary Leon Panetta and other top officials have delivered a string of private messages to Israeli leaders warning about the dire consequences of a strike. The U.S. wants Israel to give more time for the effects of sanctions and other measures intended to force Iran to abandon its perceived efforts to build nuclear weapons.Stepping up the pressure, Mr. Obama spoke by telephone on Thursday with Israeli Prime Minister Benjamin Netanyahu, and U.S. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, will meet with Israeli military officials in Tel Aviv next week.

WSJ also reported the U.S. has ramped up efforts to safeguard its facilities in the region to prepare for a possible violent response if Israel carries out an airstrike. Officials are reportedly concerned about the U.S. Embassy in Baghdad, where up to 15,000 diplomats, federal employees and contractors will remain even after the recent withdrawal of troops.